1. Technical Field
This disclosure is directed to exhaust systems for internal combustion engines that include reductant dosing systems. More specifically, the disclosed reductant dosing systems are less prone to pressure pulsations and can operate at high altitudes and at high temperatures without significant vaporization or boiling of the reductant fluid.
2. Description of the Related Art
Internal combustion engines include diesel, gasoline, gaseous fuel-powered and other engines known in the art. These engines produce a complex mixture of emissions. These emissions include gaseous compounds such as oxides of nitrogen, NO and NO2, or collectively, NOx. In atmospheric chemistry, the term NOx means the total concentration of NO and NO2. Due to increased environmental awareness, the amount of NOx emitted to the atmosphere by an engine is regulated depending on the type, size and/or class of the engine.
In order to comply with the regulation of NOx emissions, some engine manufacturers have implemented a strategy called selective catalytic reduction (SCR). SCR is an exhaust treatment process where a reductant, most commonly urea ((NH2)2CO) or a water/urea solution, is selectively injected into the exhaust gas stream of an engine and adsorbed onto a downstream substrate. The adsorbed urea decomposes into ammonia (NH3), which reacts with NOx in the exhaust gas to form water (H2O) and diatomic nitrogen (N2).
Reductant dosing systems may be used to introduce the reductant, e.g., urea, into the exhaust stream. In one example, an aqueous urea solution may be stored in a tank. The aqueous urea solution or reductant fluid may be referred to as DEF (diesel exhaust fluid), and may consist of about 32.5 wt % urea and about 67.5 wt % water.
As the power system or engine operates and produces exhaust, DEF is pumped from the tank and intermittently sprayed into the exhaust stream via an injector. Some reductant dosing systems control the DEF pressure by varying the pump speed while simultaneously monitoring the DEF pressure. However, such a reductant dosing system can produce pressure pulsations, which cannot be accurately controlled by modifying the pump speed. Further, the pressure drop in the pump suction line in combination with operation at high altitudes (low ambient pressure) and/or high temperatures can result in significant vaporization or boiling of the DEF. Vaporization or boiling of the DEF may result in a mixture of vapor and fluid entering the pump, which prevents accurate dosing of the DEF into the exhaust stream.
CN 101818675 avoids relying upon modifying the pump speed by eliminating the pump altogether and pressurizing the DEF tank. While, such a system without a pump is applicable to passenger vehicles with diesel engines, reliance upon a pressurized DEF tank alone, without a pump, may not be suitable for larger diesel power systems, such as those for trucks, generators, construction equipment, mining equipment, etc.